Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD). A closely related information collection technique is “permanent monitoring”.
In wireline logging, a sonde is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long wireline cable that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated, thereby enabling measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations.
In permanent monitoring, sensing instruments are installed in a borehole for long-term monitoring of the downhole conditions. Such instruments must survive in a hostile environment for at least months if not years, and in most cases they are not accessible for repair or replacement. The instruments can be used to measure parameters in the borehole environment or in the formation surrounding the borehole.
One of the many formation parameters of interest is conductivity (or equivalently, resistivity). Many approaches to measure the resistivity of a formation downhole rely on the measurement of varying magnetic fields. The most familiar tool that works in this way is the electromagnetic induction sonde. This sonde combines both a transmitter coil and a receiver coil. The transmitter coil converts a varying electrical current into a varying magnetic field. This field excites eddy currents in the formation, which in turn generate varying (secondary) magnetic fields. Both the primary and the secondary magnetic fields present at the receiver coil location cause a time-varying electromotive force in the receiver coil. This voltage signal is received by the tool's electronics for further processing. The intensity and phase information of the received signal, analyzed in relation to the source signal, permit a determination of the resistivity of the formation in the zone interrogated by the tool. Because a varying magnetic field also implies a varying electric field, this approach is generally described as an electromagnetic (EM) interrogation method. Unfortunately, it may be difficult to provide receiver electronics of sufficient sensitivity that can survive a downhole environment for an extended period of time.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.